HUNTSVILLE, Ala. — A lockdown has been lifted at an Alabama military installation, though officials say they are still investigating a possible active shooter incident there.

Christopher Colster, a spokesman for Redstone Arsenal in Hunstville, said Tuesday that officials do not believe there is an ongoing threat. No injuries or arrests have been reported.

Colster says a training exercise was ongoing that was supposed to end Wednesday. Officials had said on Facebook that an active shooter training exercise was planned for this week. But Colster would not say whether the active shooter report was part of that exercise, adding: “I really can’t say at this point what happened. I’m not sure myself.”

All entrances and exits at the US Army’s Redstone Arsenal were closed off, and those on base were ordered to shelter in place.

An ambulance service was sent to the facility, said Kristin Clark, a dispatcher for Huntsville Emergency Medical Services.

Redstone Arsenal spokesman Christopher Colster said in a live broadcast that the base that adjoins the city of Huntsville received reports of a person possibly armed with a weapon at a building on the base.

Colster said there were no confirmed casualties and that officials hoped workers could return to normal activities within a couple of hours.

The governor’s office says Alabama Gov. Kay Ivey is being updated about the situation, but her office couldn’t provide any details.

Ivey issued a statement Tuesday saying, “My office is aware of the developing situation at Redstone Arsenal, and we are monitoring it closely. I am praying for a swift resolution to this matter.”

More than 30,000 government employees, civilians and contractors work daily at Redstone. It is home to Army missile defense programs and other military offices.

NASA’s Marshall Space Flight Center, which oversees rocket propulsion operations, also is located at Redstone.